BEIJING (AP) — China rushed to keep an oil spill from reaching international waters Tuesday, while an environmental group tried to assess if the country’s largest reported spill was worse than has been disclosed.

Crude oil started pouring into the Yellow Sea off a busy northeastern port after a pipeline exploded late last week, sparking a massive 15-hour fire. The government says the slick has spread across a 70-square-mile stretch of ocean.

Images of 100-foot-high flames shooting up near part of China’s strategic oil reserves drew the immediate attention of President Hu Jintao and other top leaders. Now the challenge is cleaning up the greasy brown plume floating off the shores of Dalian, once named China’s most livable city.

The environmental group, Greenpeace China, shot several photographs at the scene Tuesday before their team was forced to leave. They showed oil-slicked rocky beaches, a man covered in thick black sludge up to his cheekbones, and workers carrying a colleague covered in oil away from the scene. His condition was not known.

Activists said it was too early to tell what impact the pollution might have on marine life.

In a stroke of awkward timing, meanwhile, Dalian’s International Beach Culture Festival, which draws thousands of tourists every year, started over the weekend, but the state-run Xinhua News Agency said waters around the beach had not been affected by the slick.

Officials told Xinhua they did not yet know how much oil had leaked, but China Central Television reported that no more pollution, including oil and firefighting chemicals, had entered the sea Tuesday. It was not clear how far the spill was from China’s closest neighbor in the region, North Korea.

Dalian’s vice mayor, Dai Yulin, told Xinhua that 40 specialized oil-control boats would be on the scene by Tuesday evening, along with hundreds of fishing boats. Oil-eating bacteria were also being used in the cleanup.

“Our priority is to collect the spilled oil within five days to reduce the possibility of contaminating international waters,” he said.

But an official with the State Oceanic Administration has warned the spill will be difficult to clean up even in twice that amount of time.

The Dalian port is China’s second largest for crude oil imports, and last week’s spill appears to be the country’s largest in recent memory.

“In terms of what is known to the public, this is definitely the biggest,” said Yang Ailun, spokeswoman for Greenpeace China.

“Government and business leaders have been telling the media that there’s no environmental impact. From Greenpeace’s perspective, that’s very irresponsible,” she added. “It’s too early to tell. Oil is still floating around.”

The cause of the blast was still not clear Tuesday. The pipeline is owned by China National Petroleum Corp., Asia’s biggest oil and gas producer by volume.

Chinese media continued to report Tuesday on the initial explosion and the oil cleanup. The government is known to censor reporting when an issue becomes too sensitive.

While the Chinese public has not seized on the accident as its own version of the massive BP spill in the United States, warnings over the country’s increasing dependence on oil were clear.

The International Energy Agency said Tuesday that China has overtaken the United States as the world’s largest energy consumer, using the equivalent of 2.252 billion tons of oil last year. China immediately questioned the calculation.

“China was a spectator of the Gulf of Mexico incident, but suddenly it itself has attracted attention from the whole world,” Sima Pingbang, the executive chief-editor of environmental protection website chinaepr, wrote Monday of the explosion and spill. “Chinese can no longer live above such things.”

The slick of surface oil in the Gulf of Mexico spill was 2,700 square miles late last week, far dwarfing the China spill.

But Ms. Yang worried that the scope of the Dalian spill was worse than reported so far.